1. Field of the Invention
The field of art to which this invention pertains is hydrocarbon separation. More specifically this invention relates to a process for separating olefins from a hydrocarbon feed mixture containing olefins and saturates which process employs a zeolitic adsorbent.
2. Description of the Prior Art
The treating of crystalline aluminosilicates with a caustic solution to modify certain of its properties has been recognized in the prior art. U.S. Pat. No. 3,326,797 for example, discloses a process for aqueous caustic treating of high silica zeolites having silica over alumina ratios between about 6 and 12, at treating conditions, for the sole purpose of removing a certain percentage of structural silica from the zeolite. The caustic treating, at conditions to preferably retain a final SiO.sub.2 /Al.sub.2 O.sub.3 ratio greater than about 5.5, is found to increase the adsorptive capacity of the zeolite and to increase its catalytic activity. The caustic treating process of that reference patent is concerned only with etching or leaching of silica from the zeolite structure to achieve these characteristics and neither discloses nor suggests the addition of alkali metal cations to the zeolite structure during the treating process for any reaction whatever.
Other closely related prior arts are U.S. Pat. Nos. 3,265,750 by D. W. Peck et al and 3,717,572 by A. M. deGramont. The first patent relates to a process for the separation of olefins from paraffins and discloses that the adsorbent employed could be treated by various methods, such as treatment with an amine, to inhibit polymerization. The patent makes no reference to producing an adsorbent having an increased capacity for olefins. More specifically, that reference neither teaches nor suggests our treatment step of a precursor mass comprising a type X zeolite and an amorphous binder to increase the sodium content of the zeolite and to remove a small quantity of silica and alumina thereby producing an adsorbent having both decreased catalytic activity and increased capacity for olefins.
U.S. Pat. No. 3,717,572 discloses a method of neutralizing the acidity of molecular sieves, used to separate olefin-containing mixtures, thereby reducing isomerization and polymerization activity of the molecular sieves. The neutralization is effected by immersing the seive in a solution of an inorganic base. The solvent may be water or an organic solvent but an organic solvent is preferred, and in particular methanol, since in such case a complete neutralization of the sieve is obtained with a smaller consumption of base while maintaining a "practically unchanged adsorptive capacity of the sieve." The use of water as a solvent in the neutralization method of that reference has the disadvantage of lowering the adsorptive capacity of the sieve.
We have discovered that an adsorbent having both increased capacity for olefins and decreased catalytic activity for polymerization and isomerization can be prepared by the steps of: contacting a precursor mass comprising a type X zeolite and an amorphous binder haing a Na.sub.2 O/Al.sub.2 O.sub.3 ratio of less than about 0.7 with an aqueous sodium hydroxide solution at ion-exchange conditions to increase the sodium cation content to a Na.sub.2 O/Al.sub.2 O.sub.3 ratio of greater than about 0.7 and to remove not more than about 15 wt. % of SiO.sub.2 and Al.sub.2 O.sub.3 from the precursor mass; washing the mass with water to remove excess sodium hydroxide solution; and, at least partially dehydrating the mass at dehydrating conditions. We have found that catalytic activity of the finished adsorbent decreases in proportions to the amount of sodium cations added to the zeolite by the caustic treatment. Specifically, we have found that a sodium content, expressed as the ratio Na.sub.2 O/Al.sub.2 O.sub.3, above about 0.7 is required to produce an adsorbent having the desired properties. The sodium cation added by the ion-exchange apparently replaces acid sites within the zeolite that catalyze isomerization and polymerization reaction. The removal of a small amount of silica and alumina from the precursor mass results in improved capacity of the adsorbent for olefins.
Employing the adsorbent so produced in a olefin separation process results in an improved olefin separation process because less adsorbent is required due to the adsorbent's increased capacity and because the adsorbent has a larger effective on-stream life due to its reduced cataytic activity.